Classes, properties, and methods
In object-oriented programming, you create reusable classes to perform application processing. These classes include properties and methods that define the class's behavior. To perform application processing, you create instances of these classes. PowerBuilder implements these concepts as follows:
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Classes
PowerBuilder objects (such as windows, menus, window controls, and user objects)
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Properties
Object variables and instance variables
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Methods
Events and functions
The remaining discussions in this chapter use this PowerBuilder terminology.
Fundamental principles
Object-oriented programming tools support three fundamental principles: inheritance, encapsulation, and polymorphism.
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Inheritance
Objects can be derived from existing objects, with access to their visual component, data, and code. Inheritance saves coding time, maximizes code reuse, and enhances consistency. A descendant object is also called a subclass.
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Encapsulation
An object contains its own data and code, allowing outside access as appropriate. This principle is also called information hiding. PowerBuilder enables and supports encapsulation by giving you tools that can enforce it, such as access and scope. However, PowerBuilder itself does not require or automatically enforce encapsulation.
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Polymorphism
Functions with the same name behave differently, depending on the referenced object. Polymorphism enables you to provide a consistent interface throughout the application and within all objects.
Visual objects
Many current applications make heavy use of object-oriented features for visual objects such as windows, menus, and visual user objects. This allows an application to present a consistent, unified look and feel.
Nonvisual objects
To fully benefit from PowerBuilder's object-oriented capabilities, consider implementing class user objects, also known as nonvisual user objects:
Standard class user objects
Inherit their definitions from built-in PowerBuilder system objects, such as Transaction, Message, or Error. The nvo_transaction Transaction object in the Code Examples sample application is an example of a subclassed standard class user object. Creating customized standard class user objects allows you to provide powerful extensions to built-in PowerBuilder system objects.
Custom class user objects
Inherit their definitions from the PowerBuilder NonVisualObject class. Custom class user objects encapsulate data and code. This type of class user object allows you to define an object class from scratch. The u_business_object user object in the Code Examples sample application is an example of a custom class user object. To make the most of PowerBuilder's object-oriented capabilities, you must use custom class user objects. Typical uses include:
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Global variable container
The custom class user object contains variables and functions for use across your application. You encapsulate these variables as appropriate for your application, allowing access directly or through object functions.
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Service object
The custom class user object contains functions and variables that are useful either in a specific context (such as a DataWindow) or globally (such as a collection of string-handling functions).
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Business rules
The custom class user object contains functions and variables that implement business rules. You can either create one object for all business rules or create multiple objects for related groups of business rules.
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Distributed computing
The custom class user object contains functions that run on a server or cluster of servers.
For more information, see Part 6, "Distributed Application Techniques".
